This invention relates to cerium-doped radiation-shielding glasses in the SiO.sub.2 -PbO-alkali metal oxide system, and especially to such glasses having a high absorption coefficient for high-energy X-ray and/or gamma radiation as well as neutron radiation. Radiation resistance against discoloration is provided by the cerium oxide.
Radiation-shielding glasses are utilized in radiation-shielding windows employed in various facilities, e.g., research installations, employing or separating radioactive isotopes, and reprocessing plants. These windows, made of different radiation-shielding glasses, must satisfy certain requirements:
(a) shielding of radiation from the hot cells with respect to the observer in front of the window (biological protection) down to a legally determined minimal dose [mrem]; PA1 (b) maximally high transparency of the total window even with radiation exposure over a time period of several decades (radiation resistance); PA1 (c) high stability of the glasses of the window against electrical discharge (discharge stability). PA1 (1) transparency of individual panes, PA1 (2) thickness of individual panes, PA1 (3) light losses due to scattering on surfaces of the individual panes, PA1 (4) number of panes affecting the interfaces in (3) , PA1 (5) radiation resistance of types of glasses used. PA1 T. W. Eckels and D. P. Mingesz, Hot. Lab. Proceedings, Argonne Nat. Lab., 1970. PA1 T. W. Eckels and D. P. Mingesz, Hot Lab. Proceedings, Argonne Nat. Lab., Proceedings of 18th Conference of Remote. PA1 (a) a markedly visible blue flash and/or PA1 (b) the quasi "frozen-in" flash in the glass, the so-called "Lichtenberg" figure, then the discharge limit is exceeded, and one speaks of an electrical discharge.
As for (a):
Using Pb glasses with 24-75% by weight of PbO, it is possible to construct windows for any type of application by a combination of various Pb glasses of different thicknesses so that biological protection can be ensured in every instance.
As for (b):
The transparency of a window made of several glass panes of different glasses depends on:
By selecting optically homogenous radiation-shielding glasses doped with CeO.sub.2 against radiation discoloration, high transparency of a window can be ensured even after operating a hot cell over several decades. The radiation resistance of the glasses is customarily determined by irradiation tests with Co.sup.60 at varying radiation doses. By reducing the number of individual panes and by surface refining of the panes by means of leaching processes, cementing of several panes into a compound pane, and application of antireflection coats to glass surfaces, light scattering losses on surfaces can be further reduced, and the transparency of the window can be even more increased.
As for (c):
The presently utilized radiation-shielding glasses with contents of Pb between 24% and 75% by weight from the major manufacturers of radiation-shielding glasses are safe from electrical discharge merely up to 5.times.10.sup.6 rad, i.e., after this dose of ionizing radiation, a discharge (electrical discharge) is observed in accordance with the internationally conventional testing method of Eckels and Mingesz, described in the publications:
This testing method is carried out as follows:
A falling pin of 725 g is dropped in a sliding tube from a height of 38 mm onto a test cube (100 mm.sup.3) irradiated immediately beforehand (Co.sup.60). If this causes, during this procedure
Due to the discharge, the glass is destroyed and is unusable because of the then missing transparency.
The discharge limit, with data regarding the dose [rad] and test radiation [Co.sup.60 ], such as, for example, 5.times.10.sup.6 [rad, Co.sup.60 ], is determined starting with non-irradiated material, at constantly increasing doses and with interim discharge tests, and the material is characterized correspondingly.
Therefore, discharge resistance of 5.times.10.sup.6 rad means that, with Co.sup.60 irradiation below this dose, there cannot be initiation of electrical discharge by the aforedescribed impact test. Spontaneous discharges, i.e., discharge phenomena without any external influence, such as pressure or impact, are obtained only at higher doses of radiation.
Radiation-shielding glasses free of Pb and having a relatively low absorption capability with respect to X-ray and/or gamma radiation have advantageously a discharge stability of &gt;10.sup.10 (rad). For this reason, these glasses--cerium-stabilized borosilicate glasses being the norm--are utilized in radiation-shielding windows of hot cells to protect the Pb glasses against electrostatic discharge. In this connection, they serve, on the one hand, as a protective window against pressure and impacts for the Pb-containing glasses arranged therebehind, and on the other hand as a moderator for the "hot" radiation. FIG. 1 illustrates the structure, in principle.
The primary requirement to be met by radiation-shielding windows is the maximally complete absorption of radiation in the window by the glass, or the glasses. In this context, the shielding effect of the borosilicate glass with a volume proportion of about 1/4 is, however, less than 10% of the total absorption of the window. It can be seen therefrom how the design of the window is adversely affected by the "borosilicate glass" with respect to volume and weight of the window.
Conversely, U.S. Pat. No. 3,356,579 also discloses radiation-shielding glasses utilized as "single-component" glasses, as well as windows made of these glasses. These conventional, halogen-free glasses exhibit essentially the following composition (based on the batch in % by weight):
______________________________________ SiO.sub.2 41-50 PbO 30-36 CeO.sub.2 2.0-3.5 K.sub.2 O 16-21 ______________________________________
These glasses are highly resistant against discoloration and discharge after gamma irradiation up to at least 10.sup.8 Roentgen, but have unsatisfactory neutron absorption properties. They exhibit densities of between 3.1 and 3.5 g/cm.sup.3 and, with a thickness of 1 inch (=2.54 cm), have an initial transmission of above 90% at approximately 550 nm.
British Patent 764,575 discloses a gamma ray absorptive glass comprising 17% to 25% K.sub.2 O, 35% to 50% SiO.sub.2, 26% to 45% P.sub.5 O and 0.8% to 1.8% CaO.sub.2. According to this reference lanthanides (didymiumoxid) should be avoided. There is no disclosure in the reference of the glasses having a high resistance to electric discharge or that such glasses would meet the radiation qualities needed to avoid electric discharges in the glass.
In the prior art radiation-shielding glasses, compounds such as BaO and also SrO are included because they are particularly absorptive of gamma radiation. Also, amounts of Na.sub.2 O over 4 wt % are used because of the improved fluxing qualities of the glass melt. Although other bivalent compounds, e.g., MgO, CaO, ZnO and CdO are used in prior glasses, these compounds were not removed in purifying the glasses because it was believed that these compounds could be tolerated. Also, glasses with a resistance to electrical discharge of greater than 5.times.10.sup.8 rad were unknown and not producible from prior art glasses.
U.S. Pat. No. 4,520,115 discloses a glass composition for tube faces in cathode ray tubes. It is known that such glasses can contain, e.g., PbO for shielding properties against X-ray radiation, CeO.sub.2 for a high resistance to discoloration and rare earths Gd to improve electron browning behavior. However, such glasses have only poorer transmission qualities which do not matter in cathode ray tube faces which have thicknesses of only about up to 1 inch. The transmission qualities of such glasses would in general be unsuitable in radiation shielding windows having greater thicknesses, for example, up to 1 meter, and in any case, over 30 mm.